Diabetic patients can improve their life quality and life expectancy by maintaining their blood glucose concentration close to the natural level of a healthy person. To achieve this natural concentration, diabetic patients must frequently measure their glucose concentration, and adjust their insulin dosing in accordance with the measured concentration. Usually, a blood sample is obtained for measurement of blood glucose concentration, and there are a number of different glucose test kits on the market based on measurement from a blood sample. The disadvantage of these test kits is the need to take a blood sample which must be collected from a suitable place in the body.
Self monitoring devices, based on capillary blood glucose, are practical but still require repeated and frequent skin punctures, which is inconvenient for the patient and require certain hygienic precautions.
Biological sensors in the form of implantable devices are also known in the art and include electrochemical devices and optical devices based on the creation of an electrical or optical signal by the consumption of the compound detected by the analysis. An example is to be found in U.S. Pat. No. 6,011,984, which discloses methods utilising an amplification component. The sensitivity and the responsivity of such devices are influenced by the formation of a bio film, for example, by fibrous encapsulation of the device which reduces the transport rate of the compound to the sensor. Depending on the specific sensor, other mechanisms which cause deterioration of sensor performance of implanted devices, may also be present, for example, membrane de-lamination and degradation, enzyme degradation and electrode passivation.
Various proposals have been made for non-invasive measurement of glucose levels in the human body by spectroscopic methods but the effects of water in the body, the low concentration of glucose to be measured and the optical effects produced by skin all contribute to the difficulty of making satisfactory measurements.
One solution proposed (Gowda et al Proc. SPIE Vol. 4263 (2001) p. 11 et seq) has been to provide an implanted window in the skin but, leaving aside any other considerations, some patients, at least, would find this unpleasant.
Another solution (U.S. Pat. No. 5,372,135) is to perform the analysis through the ear lobe and perform a computer analysis on results before and after the volume of the blood in the tissue has been changed. This technique has the disadvantage that the skin has an effect both on the entry and the exit of light.
It is an object of the invention to provide an implantable device which can be used for monitoring analyte concentration but enables the effects of skin in the analysis to be reduced.